This sounds like blight, a fungal disease more common in damp summers like this one.

Blight hits all outdoor tomatoes in a damp, humid season. We have even lost all of ours since the start of September due to wet weather, after a dry sunny August. The only thing you can do is spray Bordeaux mixture as a preventative or grow under some form of cover. If you haven't got a greenhouse, just covering the plants with a plastic 'roof' should keep them safer for longer. The other thing I have found makes a difference is getting rid of potato 'sports' as these seem to harbour the blight spores, though spores just blow in on the wind anyway.